The shoot apical meristem consists of different functional regions that can be distinguished by the orientation of the cell division planes and by cell size and activity. The angiosperm vegetative shoot apical meristem usually has a highly stratified appearance, typically with three distinct layers of cells. These layers are designated L1, L2, and L3, where L1 is the outermost layer (Figure 16.13). Cell divisions are anticlinal in the L1 and L2 layers; that is, the new cell wall separating the daughter cells is oriented at right angles to the meristem surface. Cell divisions tend to be less regularly oriented in the L3 layer. Each layer has its own stem cells, and all three layers contribute to the formation of the stem and lateral organs.
Active apical meristems also have an organizational pattern called cytohistological zonation. Each zone is composed of cells that may be distinguished not only on the basis of their division planes, but also by differences in size and by degrees of vacuolation (see Figure 16.13B). These zones exhibit different patterns of gene expression, reflecting the different functions of each zone (Nishimura et al. 1999; Fletcher and Meyerowitz 2000).
The center of an active meristem contains a cluster of relatively large, highly vacuolate cells called the central zone. The central zone is somewhat comparable to the quiescent center of root meristems (which will be discussed later in the chapter). A doughnut-shaped region of smaller cells, called the peripheral zone, flanks the central zone. A rib zone lies underneath the central cell zone and gives rise to the internal tissues of the stem.
These different zones most likely represent different developmental domains. The peripheral zone is the region in which the first cell divisions leading to the formation of leaf primordia will occur. The rib zone contributes cells that become the stem. The central zone contains the pool of stem cells, some fraction of which remains uncommitted, while others replenish the rib and peripheral zone populations (Bowman and Eshed 2000).
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